problems with setClass or/and setMethod

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Hello,

I am working my way through "A (not so) Short introduction to S4"

I created a class

setClass(Class = "Trajectories",
        representation = representation(times = "numeric",traj = "matrix"))

and tried to build a method using

setMethod(
         f = "plot",
         signature = "Trajectories",
         definition = function(X, y, ...){
           matplot(x at times, t(x at traj), xaxt = "n", type = "l", ylab = "",
xlab = "", pch = 1)
           axis(1, at = x at times)
         }
       )

R responds with an error message:

Creating a generic function for ?plot? from package ?graphics? in the
global environment
Error in conformMethod(signature, mnames, fnames, f, fdef, definition) :
 in method for ?plot? with signature ?x="Trajectories"?: formal arguments
(x = "Trajectories", y = "Trajectories", ... = "Trajectories") omitted in
the method definition cannot be in the signature

Did anything change in the transition to R-3.0?
I doubt it worked in earlier versions. There is a misprint of "X" where there should be an "x". I'm unable to explain why the "y" is along side the "x" in the argument list since the 'definition' function does nothing with it.
is there any other, more recent introduction to S4 classes recommended?

Thank you

for your help.

Andreas

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